Electrochemical methodology has been developed for monoamine analysis in vivo at specific locations in the brain of the laboratory rat. In these methods, as in all modern electro-analytical procedures, the potential difference between a working electrode and a constant potential (reference) electrode is controlled electronically through an auxiliary electrode; the resulting current through the working electrode is directly proportional to the concentration of monoamine in extracellular fluid surrounding the electrode; the potential at which the current is observed is indicative of the type of compound being detected. These methods offer the advantage that they perform analyses; (1) in vivo; (2) specific to the subject compounds and their metabolites; (3) concurrently for these compounds; (4) instantaneously; (5) in a specific microscopic location in the brain; (6) non-destructively and repetitively; (7) selectively in the fluid phase and (8) quantitatively. Refinements in the methodology have efficiently excluded background interferences, achieved long-term electrode stability and optimized the resolution of closely related species. We have achieved in vivo electroanalytical determination of endogenous dopamine, homovanillic acid and serotonin. About seventy-five types of drug manipulation influencing chemical neurotransmission at the synaptic level have been conducted. It is proposed to continue these studies in order to: (a) determine monoamine levels following drug administration; (b) observe monoamine release in respone to electrical stimulation; (c) correlate in vivo measurements with physiological states; (d) examine the effects of nutrition on monoamine and tyrosine levels and (e) compare monoamine levels with location in the brain.